Name: ___________________________
Practice – 2.3 Time, Velocity and Speed
1. A helicopter blade spins at exactly 100 revolutions per minute. Its tip is 5.00 m from the
center of rotation.
A. Calculate the average speed of one of the blade tips in the helicopter’s frame of
reference.
B. What is its average velocity of one of the blade tips over one revolution?
2. The North American and European continents are moving apart at a rate of about 3 cm/y.
At this rate how long will it take them to drift 500 km farther apart than they are at
present?
3. Land west of the San Andreas fault in southern California is moving at an average velocity
of about 6 cm/y northwest relative to land east of the fault. Los Angeles is west of the
fault and may thus someday be at the same latitude as San Francisco, which is east of the
fault. How far in the future will this occur if the displacement to be made is 590 km
northwest, assuming the motion remains constant?
4. On May 26, 1934, a streamlined, stainless steel diesel train called the Zephyr set the
world’s nonstop long-distance speed record for trains. Its run from Denver to Chicago took
13 hours, 4 minutes, 58 seconds, and was witnessed by more than a million people along the
route. The total distance traveled was 1633.8 km. What was its average speed in km/h and
m/s?
5. Tidal friction is slowing the rotation of the Earth. As a result, the orbit of the Moon is
increasing in radius at a rate of approximately 4 cm/year. Assuming this to be a constant
rate, how many years will pass before the radius of the Moon’s orbit increases by 3.84×106 m
(1%)?
6. A student drove to the university from her home and noted that the odometer reading of
her car increased by 12.0 km. The trip took 18.0 min.
A. What was her average speed?
B. If the straight-line distance from her home to the university is 10.3 km in a
direction 25.0º south of east, what was her average velocity?
C. If she returned home by the same path 7 h 30 min after she left, what were her
average speed and velocity for the entire trip?
7. The speed of propagation of the action potential (an electrical signal) in a nerve cell
depends (inversely) on the diameter of the axon (nerve fiber). If the nerve cell connecting
the spinal cord to your feet is 1.1 m long, and the nerve impulse speed is 18 m/s, how long
does it take for the nerve signal to travel this distance?
8. Conversations with astronauts on the lunar surface were characterized by a kind of echo
in which the earthbound person’s voice was so loud in the astronaut’s space helmet that it
was picked up by the astronaut’s microphone and transmitted back to Earth. It is reasonable
to assume that the echo time equals the time necessary for the radio wave to travel from
the Earth to the Moon and back (that is, neglecting any time delays in the electronic
equipment). Calculate the distance from Earth to the Moon given that the echo time was
2.56 s and that radio waves travel at the speed of light (3.00×108 m/s) .
9. A football quarterback runs 15.0 m straight down the playing field in 2.50 s. He is then
hit and pushed 3.00 m straight backward in 1.75 s. He breaks the tackle and runs straight
forward another 21.0 m in 5.20 s. Calculate his average velocity:
A. For each of the three intervals, and
B. For the entire motion.
10. The planetary model of the atom pictures electrons orbiting the atomic nucleus much as
planets orbit the Sun. In this model you can view hydrogen, the simplest atom, as having a
single electron in a circular orbit 1.06×10−10 m in diameter.
A. If the average speed of the electron in this orbit is known to be 2.20×106 m/s,
calculate the number of revolutions per second it makes about the nucleus.
B. What is the electron’s average velocity?
Solutions:
1. A. 3.14 x 103 m/min (52.4 m/s)
B. 0
7
2. 2 x 10 y
3. 1 x 107 y
4. 124.88 km/h, 34.689 m/s
5. 1 x 108 y
6. A. 0.667 km/min (40.0 km/h)
B. 0.572 km/min @ -25.0o (34.3 km/h @ -25.0o)
C. speed = 3.20 km/h, velocity = 0
7. 6.1 x 10-2 s
8. 3.84 x 108 m
9. A. 6.00 m/s, -1.71 m/s, 4.04 m/s
B. 3.49 m/s
15
10. A. 6.61 x 10 rev/s
B. 0